Japanese Patent Laid-Open Publication No. 2005-297781 (JP 2005-297781 A) discloses a seat belt apparatus, which includes a rotation amount detection section for detecting a rotation amount of a belt reel in order to accurately determine current pay-out and take-up amounts of a belt from and to a retractor. In this disclosed seat belt apparatus, the rotation amount detection section includes a rotation sensor attached directly to the rotation shaft of the belt reel of the retractor, and thus, even though the belt reel is not always connected to a reel driving motor, current pay-out and take-up amounts of the belt can always be detected with a sufficient resolution. More specifically, the rotation amount detection section comprises a rotatable magnetic disk coupled to the shaft of the belt reel, and a magnetic sensor having two Hall ICs. Two phase-shifted pulse waveform signals are output from the two Hall ICs. Rotating direction of the belt reel can be identified in accordance with a phase shift state between the two pulse waveform signals, and a rotation amount of the belt reel can be identified in accordance with the number of pulses in either one of the two pulse waveform signals.
However, with the disclosed seat belt apparatus, which does not have a function for determining whether or not the rotation amount detection section is currently out of order or in a failed state, it is not possible to determine whether the rotation amount detection section is operating normally. Thus, when the rotation amount detection section is not operating normally, there is the possibility that erroneous determinations will undesirably take place in control operations of the seat belt apparatus.
Further, there have been known seat belt apparatus provided with a gunpowder-type pretensioner mechanism. In the event there has occurred an emergency that causes activation of the gunpowder-type pretensioner mechanism and requires operations of the seat belt apparatus beyond the resolution of the rotation amount detection, it becomes important to compulsorily inhibit the above-mentioned function for detecting a failure of the rotation amount detection section.